# [EXTERNAL] Re: rfc5280 serialNumber question.

Erwann Abalea erwann.abalea at docusign.com
Fri Jul 21 18:20:38 UTC 2023

```On 8 bits, if the highest bit is set to 1, then the value is between 0x80
and 0xFF.
In your examples, the highest octet of the integer has its highest bit set
(0xFE in the first example, 0xAE in the second), and therefore the DR
encoding added a heading 00, making the real length 9 octets (l=9 on third
column).

On Fri, Jul 21, 2023 at 8:00 PM Robert Moskowitz <rgm at htt-consult.com>
wrote:

> I looked at a couple of certs.  I might think that if the first hex is
> "F" then the 1st bit is 1, but:
>
>
>
>      8:d=2  hl=2 l=   3 cons:   cont [ 0 ]
>     10:d=3  hl=2 l=   1 prim:    INTEGER           :02
>     13:d=2  hl=2 l=   9 prim:   INTEGER           :FE0E6F3753087370
>
>      8:d=2  hl=2 l=   3 cons:   cont [ 0 ]
>     10:d=3  hl=2 l=   1 prim:    INTEGER           :02
>     13:d=2  hl=2 l=   9 prim:   INTEGER           :AEB77AEE2A3CBCD3
>
>
> I am not seeing a difference in the serialNumber field length.
>
> On 7/21/23 08:58, Robert Moskowitz wrote:
> > Per sec 4.1.2.2
> >
> >    Given the uniqueness requirements above, serial numbers can be
> >    expected to contain long integers.  Certificate users MUST be able to
> >    handle serialNumber values up to 20 octets.  Conforming CAs MUST NOT
> >    use serialNumber values longer than 20 octets.
> >
> >
> > At some point some years ago it was pointed out here that serialNumber
> > OID encoding preappends 0x00 if the first bit is a 1.
> >
> > Does this actually make the serialNumber a byte longer?  Or is this
> > only encoding?  Thus IF that first bit is a 1, obviously the OID value
> > is a byte longer.  But when the serialNumber OID is decoded is this
> > longer value returned or the original value?
> >
> >
> > I am girding up to debate an implementation where the CP says
> > serialNumber MUST be unique, and their implementation uses a 20-byte
> > SN.  I don't think they take care at all about the value of the 1st
> > byte.  I doubt in their testing to date they have generated a SN in
> > that range.
> >
> > So how does the SN with the added byte get decoded?
> >
> > thanks
> >
> >
>
>

--
Cordialement,
Erwann Abalea.
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